As a kind of imaging apparatus, digital cameras have been widely used. The digital camera images a subject through an image sensor such as a CCD image sensor. Many of the digital cameras are provided with a display device such as a LCD panel. The display device is not only used for displaying images reproduced based on captured image data, but also used as an electronic viewfinder for displaying the camera-through image. To display the camera-through image, the image sensor periodically outputs image signals at a predetermined frame rate, and the image frames obtained through the image sensor are sequentially displayed on the display device. Thus, the display device displays a moving image of a subject that exists in a shooting field of the image sensor in a real time fashion. So the camera user can frame and shoot a scene while observing the camera-through image.
Generally, the display device is driven asynchronously with the image sensor. To display the camera-through image, the image data obtained through the imaging by the image sensor is temporarily written in an image memory and, thereafter, the image data is read out of the image memory and is sent to the display device in synchronism with driving timing of the display device.
As a method of driving the display device and the image sensor asynchronously, Japanese Laid-open Patent Application No. Hei 6-124073 teaches using a couple of image memories having a capacity of storing image data of one frame each. According to this prior art, image data as transferred from an external device is written in one of the two image memories, while the image data written in the other image memory is being read out to display image frames at a frame rate of 60 frames per second on the display device. Each time the image data of one frame is completely read out of one memory, the two image memories alternate their rolls for writing and reading the image data.
Japanese Laid-open Patent Application No. Hei 11-296155 discloses a technique using first to third frame buffer memories as image memories. According to this prior art, one of the three buffer memories is used for writing the supplied image data, while other two buffer memories are used for reading the image data. When the writing of the image data of one frame completes in the first buffer memory, image data of another frame starts being written in the second buffer memory. In a period from the completion of writing the image data in the first buffer memory to the start of reading the image data from the first buffer memory, image data having been written in the third butter memory is used for displaying an image frame on a display device. Thus, even where the rate of writing the supplied image data differs from the rate of reading the image data, the respective frame buffer memories avoid being double-accessed for writing and reading the image data at the same time.
However, according to the above-mentioned methods of writing and reading image data for displaying an image, there is a time lag of more than one frame period from a time when an image frame of a subject is obtained by an image sensor to a time when the obtained image frame is displayed on a display device. Therefore, these prior arts have a problem when they are applied to a digital camera for displaying the camera-through image, that the camera user cannot exactly capture a picture at an expected moment because of the time lag between the actual movement of the subject and the camera-through image. Such a delay in shutter timing cannot be negligible when the subject moves quickly.